Vascular smooth muscle is characterized by its capacity to develop high isometric force and by low maximum shortening velocities compared to skeletal muscle These mechanical responses are the output of a contractile system with a high ratio of actin:myosin and a total myosin content approximately one-fifth that of skeletal muscle. Smooth muscle contractile proteins are also distinctive in their high solubility at low ionic strengths and in exhibiting Mg ions-activated actomyosin ATPase activity which is very low compared to the CA ions-activated ATPase of the myosin alone. The objectives of this project are to characterize the contractile proteins and their interactions in votro, and to relate these properties to the specific mechanical behavior of smooth muscle. Biochemical studies will be made of the actin-myosin interaction using purified preparations of myosin and its proteolytic subfragments, heavy meromyosin, and subfragment 1. The interaction will be tested by kinetic studies of the ATPase activity and sedimentation behavior. Myosin-myosin interactions and factors affecting filament formation will be examined by electron microscopic methods. Biophysical studies of isolated smooth muscle preparations will be conducted to characterize the mechanical behavior of the tissue. Most of this work will be conducted using smooth muscle from the media of the hog carotid artery. However, some comparative studies will be made of contractile protein content, ATPase activity, and the physical properties of actin and myosin from smooth muscle of uterine, visceral, and other tissues to determine whether there are differences in the contractile proteins. Similar studies are proposed for smooth muscle of various arteries of differing sizes to determine if data obtained from the carotid is applicable to other vessels.